Between his first book, the novel Prelude, published in 1920, and his last, a book of poetry, Twilight, published in 1982, Nichols wrote more than 60 books and plays. Besides novels, mysteries, short stories, essays and children's books, he wrote a number of nonfiction books on travel, politics, religion, cats, parapsychology, and autobiography. He wrote for a number of magazines and newspapers throughout his life, the longest being weekly columns for the London Sunday Chronicle newspaper (1932–1943) and Woman's Own magazine (1946–1967).

Nichols is now best remembered for his gardening books, the first of which, Down the Garden Path, was illustrated, as were its two sequels, by Rex Whistler. The bestseller, which has had 32 editions and has been in print almost continuously since first published in 1932, was the first of his trilogy about Allways, his Tudor thatched cottage in Glatton, Cambridgeshire. The books are written in a poetic manner, with a rich, creative language, evoking emotional and sensual responses but also with a lot of humour and even a hint of irony.[1] They were parodied by W. C. Sellar and R. J. Yeatman in Garden Rubbish (1936), where the Nichols figure was called "Knatchbull Twee".

A book about his city garden, near Hampstead Heath in London, Green Grows the City, published in 1939, was also very successful. The book introduced Arthur R. Gaskin, who was Nichols's manservant from 1924 until Gaskin's death in 1966. Gaskin was a popular character, who also appeared in the succeeding gardening books.

A later trilogy written between 1951 and 1956 documents Nichols's travails renovating Merry Hall (Meadowstream), a Georgian manor house in Agates Lane, Ashtead, Surrey, where Nichols lived from 1946 to 1956. The books often feature his gifted but laconic gardener "Oldfield". Nichols's final trilogy is referred to as "The Sudbrook Trilogy" (1963–1968) and concerns his late 18th-century attached cottage at Ham, near Richmond, Surrey.

Nichols wrote on a wide range of topics, always looking for "the next big thing". As examples, he ghostwroteDame Nellie Melba's 1925 "autobiography" Memories and Melodies (he was at the time her personal secretary, and his 1933 book Evensong was believed based on aspects of her life).[2] In 1966 he wrote A Case of Human Bondage about the marriage and divorce of writer William Somerset Maugham and his interior-decorator wife, Syrie, which was highly critical of Maugham. Father Figure, which appeared in 1972 and in which he described how he had tried to murder his alcoholic and abusive father, caused a great uproar and several people asked for his prosecution. His book about spiritualism was not well received, which disappointed him.